The present invention describes a method to produce large ingots of silicon carbide (SiC). The method can be tailored to produce polycrystalline SiC, or crystalline SiC of any polytype.
Crystalline silicon carbide is a semiconductor material which is useful for the fabrication of silicon carbide devices which can operate at higher power and higher temperatures when compared to traditional semiconductor crystalline materials such as silicon and gallium arsenide.
Bulk crystalline SiC is a difficult material to grow since the growth process requires extremely high temperatures (1900-2500° C.). The most common method for the growth of bulk crystalline SiC is sublimation.
The benchmark art for the growth of bulk crystalline SiC in commercial applications is that of Davis, et. al. U.S. Pat. RE34,861 which describes a sublimation method. The method therein emerged as an improvement on methods previously reported by Lely (U.S. Pat. No. 2,854,364), Tairov (see J. Crystal Growth, 52 (1981) pp. 46) and many others.
Sublimation reactions for bulk crystalline SiC growths are commonly conducted in a closed cylindrical reaction cell in which is located a furnace. The heat source for the reaction can be resistive heating or RF induction heating. The reaction cell is typically made of graphite. A seed crystal is placed inside the cell, usually at the top and the source material for growth of the crystal is placed opposite the seed. Upon heating the reaction cell, the source material is vaporized and it then condenses on the seed crystal.
While the process described above is relatively simple to envision, in practice it is hard to realize. The control of the reaction and vapor transport depends on a plurality of variables, none of which are directly under the users control during the process. The process cannot be monitored in-situ as it is performed in a closed, opaque cell. The transport of vapor from the source to the seed, and its condensation and crystallization is significantly influenced by the temperature distribution in the cell. As material vaporizes and transports from the source region to the seed region the temperature distribution changes. All of these variables need to be coordinated a priori to execution to achieve successful crystal growth.
Typically a powder form of silicon carbide is used as the source material. Selection of the particle size and its distribution will impact the growth process. Davis, et. al., teach that the powder should be of constant polytype composition “which are made up of a constant proportion of polytypes, including single polytypes.” In doing so this maximizes the repeatability of the vapor composition. Wang, et. al. (J. Crystal Growth (2007) doi:10.1016/j.jcrysgro.2007.03.022) and the references therein show that SiC powder particle size geometries effect packing and different SiC particle sizes can be used to increase the crystal growth rate.
The purity of the SiC powder will impact the purity of the SiC crystal and in turn, affect its resistivity value and conduction of electricity. Low cost, high purity silicon carbide powders are not readily available. Specific control of the reaction environment (temperature, pressure, etc) during sublimation SiC crystal growth is required to limit incorporation of undesired impurities from the source into the SiC crystal. Example impurities of concern in SiC crystal growth for semiconductor device applications include boron, phosphorous, nitrogen, aluminum, titanium, vanadium, and iron.
Ota, et. al. (Materials Science Forum Vols. 457-460 (2004) p. 115) show the growth of high resistivity SiC crystals using mixtures of pure silicon and carbon powder can be achieved by variation of the relative amounts of silicon and carbon in the powder mixture. They report the results using a source based on mixture of silicon and carbon powder mixtures, and they discuss sintered the mixture gives better results compared to a SiC powder source. The silicon and carbon powder sources show reduced incorporation of boron and aluminum. This work does not offer insight to the repeatability of their method.
Other variants of source materials for SiC crystal growth have been reported. Balakrishna, et. al. (U.S. Pat. No. 5,985,024) developed a method to reduce incorporation of impurities in SiC crystal growth by using a novel SiC in-situ source method wherein high purity silicon metal is vaporized in the presence of high purity hydrocarbon gas to deliver the species for silicon carbide growth. Maruyama, et. al. (U.S. Pat. No. 7,048,798 B2) describes a method for the formation of SiC source powder material by reaction of organosilicon materials and carbon containing resins.
Without control of the SiC powder size and packing, a lack of repeatability of the SiC crystal growth process will occur. Purity control of source materials is important to yield high purity SiC crystals. More efforts are required to create alternatives to common SiC powder that are low in impurities and useful for the growth of SiC crystals. A repeatable process to grow SiC crystals suitable for use in semiconductor device applications requires a delicate balance and management of all these variables.